Charlie wrote:

Does that graph say what I think it says? if it does, then the weakest & most
dangerous link in the GPS chain just went away with SA. I think we all could
(literally) live with 30 feet of altitude error.

Am I interpreting correctly?

Paul responded:

Yes. Now that you mention it that is a marked improvement in altitude
reporting. I would say you are right and if you can't see the runway
at 30 feet up you should go someplace else.

Russell Kent wrote:

Not that simple.  The graph shows the measured GPS altitude error from causes such
as atmospheric effects, yadda yadda.  Keep in mind that GPS measures your altitude
as referenced to the World Geodetic Survey 1984 (WGS84) reference ellipsoid.  Your
GPS unit then looks up your measured latitude and longitude in an internal table
that has charted variations from the reference ellipsoid (i.e. the real ground is
usually higher or lower than the reference ellipsoid).  Then the GPS unit can
display your computed altitude over the ground.  If the GPS unit's chart datum is
incorrect, you could be in a world of hurt.  This is part of the FAA qualification
procedure, and part of the reason why Jeppesen charts are so bloody expensive.

A backpacker's GPS may not have good chart datum.  Be careful!

Russell Kent

Ok after reading it a couple of times I think I understand. In other
words nobody really knows the exact altitude of your runway. I have run into
this before with FAA airport lat/long data. Many runway's exact locations are
unknown.
Nobody has come out from an agency and surveyed those runways for position or
altitude. Is that about right Russell? 

The best way around this is to record the GPS information for your home airport 
so you know exactly where and how high it is. Won't help if you go someplace 
else however.
 
Paul Lamar
 
The Aircraft Rotary Engine Newsletter.              Powered by Linux.

http://home.earthlink.net/~rotaryeng/            http://www.linux.org